In the related art, there is a need for detecting ultraweak fluorescence in biological systems research, particularly in research markets in which microscopes are used, and there are known scanning laser microscopes that are equipped with a photomultiplier tube (PMT) serving as a detector (for example, see Patent Literature 1). In addition, since a PMT suffers from the problem that it is degraded or damaged when irradiated with an excessively high level of light, one known approach involves stopping the operation of the PMT when a signal of a certain value or greater is output from the PMT for a prescribed duration due to strong incident light, by means of a protection circuit for preventing degradation or damage due to an excessively high level of light.